Carbon-fiber web structure type field emitter electrode and fabrication method of the same

ABSTRACT

The present invention provides a field emitter electrode and a method for fabricating the same. The method comprises the steps of mixing a carbonizable polymer, carbon nanotubes and a solvent to prepare a carbon nanotube-containing polymer solution, electrospinning (or electrostatic spinning) the polymer solution to form a nanofiber web layer on a substrate, stabilizing the nanofiber web layer such that the polymer present in the nanofiber web layer is crosslinked, and carbonizing the nanofiber web layer such that the crosslinked polymer is converted to a carbon fiber.

RELATED APPLICATIONS

The present application is based on, and claims priority from, KoreanApplication NO. 2004-76836, filed on Sep. 24, 2004, the disclosure ofwhich is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a carbon nanotube field emitterelectrode. More particularly, the present invention relates to a fieldemitter electrode having a novel structure which not only improves theadhesive strength of carbon nanotubes but also exhibits improved contactresistance, and a method for fabricating the field emitter electrode.

2. Description of the Related Art

Generally, field emission devices are light sources for emittingelectrons in a vacuum environment, and use the principle that electronsemitted from fine particles are accelerated by a strong electric fieldto collide with fluorescent substances, thus emitting light. Such fieldemission devices provide superior luminescence efficiency and arecompact and lightweight, compared to light sources for generalilluminators, such as incandescent lamps. In addition, since fieldemission devices do not use heavy metals, unlike fluorescent lamps, theyhave the advantage of being environmentally friendly. For these reasons,field emission devices have drawn attention as next-generation lightsources for various illuminators and display devices.

The performance of field emission devices is mainly determined byemitter electrodes capable of emitting a field. In recent years, acarbon nanotube (CNT) has been widely used as an electron-emittingmaterial for emitter electrodes having excellent electron emissionproperties.

However, carbon nanotubes have a problem in terms of non-uniform growthon a large-area substrate. In an attempt to solve this problem, carbonnanotubes grown by a separate process are purified before adhesion tothe substrate. Representative methods for fabricating a carbon nanotubeemitter electrode include common printing and electrophoresis.

The fabrication of a carbon nanotube emitter electrode by printing iscarried out in accordance with the following procedure. First, anelectrode material is coated on a smooth substrate to form an electrodelayer. Then, a paste of carbon nanotubes and a silver powder is printedon the electrode layer. The resulting structure is subjected to anannealing process to remove the resin and the solvent present in thepaste. The annealed structure is subjected to taping to partially exposethe carbon nanotubes to the surface.

However, the conventional method has the problems that the procedure iscomplicated and uniform dispersion of the carbon nanotubes is difficult,thus deteriorating the characteristics of the final field emitterelectrode. In addition, sufficient physical and mechanical adherence ofthe paste to the underlying electrode material cannot be achieved byknown paste printing processes.

On the other hand, the fabrication of a carbon nanotube emitterelectrode by electrophoresis is carried out by the following procedure.Referring to FIG. 1, first, previously purified carbon nanotubes and adispersant (e.g., a cationic dispersant) are mixed in an electrolyticsolution. Then, a predetermined voltage is applied to both electrodesimmersed in the electrolytic solution to adhere the carbon nanotubes toa substrate formed on the cathode.

The use of electrophoresis enables the carbon nanotubes to be relativelyuniformly dispersed in the electrolytic solution, and simplifies theoverall procedure. However, the conventional method has the problem thatthe carbon nanotubes are susceptible to mechanical impact because oftheir poor adhesive strength to the substrate.

In addition, since a large quantity of organic components remain on theelectrically conductive polymer constituting the emitter electrode, theyare likely to be oxidized once the emitter electrode is operated, whichlargely deteriorates the field emission properties. In extreme cases,undesired gases may be generated in an electron emission space where avacuum is required, seriously degrading the performance of the fieldemission device.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide anovel field emitter electrode having a carbon-fiber web structure inwhich carbon nanofibers contain carbon nanotubes.

It is another object of the present invention to provide a method forfabricating the field emitter electrode by electrospinning (orelectrostatic spinning), the field emitter electrode having acarbon-fiber web structure in which carbon nanofibers contain carbonnanotubes.

In accordance with one aspect of the present invention, the aboveobjects can be accomplished by a field emitter electrode having acarbon-fiber web structure comprising a carbon-fiber web layer composedof a plurality of carbon nanofibers, and a plurality of carbon nanotubesadhered to or contained in the plurality of nanofibers wherein at leasta portion of the carbon nanotube is exposed to the outside of thenanofiber.

The carbon-fiber web layer can be formed of at least one materialselected from the group consisting of polyacrylonitriles, celluloses,phenol resins, and polyimides.

Preferably, the carbon nanofibers have a larger diameter than the carbonnanotubes.

In accordance with another aspect of the present invention, there isprovided a method for fabricating the field emitter electrode having acarbon-fiber web structure by electrospinning, the method comprising thesteps of: mixing a carbonizable polymer, carbon nanotubes and a solventto prepare a carbon nanotube-containing polymer solution;electrospinning the polymer solution to form a nanofiber web layer on asubstrate; stabilizing the nanofiber web layer such that the polymerpresent in the nanofiber web layer is crosslinked; and carbonizing thenanofiber web layer such that the crosslinked polymer is converted to acarbon fiber.

The carbonizable polymer may be at least one selected from the groupconsisting of polyacrylonitriles, celluloses, phenol resins, andpolyimides.

The nanofiber web layer can be composed of nanofibers having a diameterlarger than the carbon nanotubes. The substrate may be an electricallyconductive substrate, such as an aluminum or copper sheet.

The stabilization of the nanofiber web layer can be carried out byoxidizing the polymer present in the nanofiber web layer in an oxidizingatmosphere at 150˜350° C. The carbonization of the nanofiber web layercan be carried out by carbonizing the crosslinked polymer in an inertatmosphere at 600˜1,300° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic views of an apparatus used in a conventionalmethod for fabricating a field emitter electrode by electrophoresis;

FIG. 2 is a flow chart illustrating a method for fabricating a fieldemitter electrode according to the present invention;

FIG. 3 is a schematic view of an electrospinning apparatus usable in amethod for fabricating a field emitter electrode according to thepresent invention;

FIGS. 4 a to 4 e are scanning electron micrographs (SEM) (5,000×) ofelectrospun nanofiber web structures formed in Example 1 of the presentinvention;

FIG. 5 is a SEM (80,000×) of a nanofiber web structure formed in Example1 of the present invention;

FIGS. 6 a to 6 c are photographs of a carbon-fiber web structure formedin Example 1 of the present invention taken at different magnifications;and

FIG. 7 is a photograph showing the luminescent state of a field emissiondevice to which a field emitter electrode having a carbon-fiber webstructure formed in Example 1 of the present invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in more detail withreference to the accompanying drawings.

FIG. 2 is a flow chart illustrating a method for fabricating a fieldemitter electrode according to the present invention.

Referring to FIG. 2, the method for fabricating a field emitterelectrode according to the present invention is initiated by mixing acarbonizable polymer, carbon nanotubes and a solvent to prepare a carbonnanotube-containing polymer solution (S13).

Any polymer can be used in the present invention so long as it iscarbonizable. The polymer may be at least one material selected from thegroup consisting of polyacrylonitriles, celluloses, phenol resins, andpolyimides. The kind of the solvent used in the present invention can beproperly selected according to the type of the selected polymer. Forexample, the solvent may be dimethylformamide (DMF), toluene, benzene,acetone, or alcohol.

The carbon nanotubes used in the present invention can be obtained bypulverizing multi-wall or single-wall carbon nanotubes by chemical vapordeposition (CVD) or arc discharge, and purifying the pulverizedmulti-wall or single-wall carbon nanotubes by known processes, such asfield-flux-flow fractionation. The carbon nanotubes thus obtainedpreferably have a length of 1 μm to 2 μm, and a diameter of a fewnanometers to tens of nanometers.

Thereafter, the polymer solution is electrospun to form a nanofiber weblayer on a substrate (S15).

Generally, electrospinning is a process that has been widely used invarious industrial fields, including fibers, fuel cells, and cellelectrodes, and refers to a process wherein nanofibers having a diameterof a few nanometers to hundreds of nanometers are spun from a polymericprecursor material by applying an electrostatic voltage to the precursorto form a random web structure. Similarly, when an electrostatic voltageis applied between the carbon nanotube-containing polymer solution andthe substrate in step (S15), nanofibers containing the carbon nanotubesare spun on the substrate and thus a desired nanofiber web layer can beformed on the substrate.

The substrate usable in step (S15) may be a non-conductive film having athickness small enough to allow the applied electrostatic voltage forelectrospinning, but is preferably an electrically conductive substrate,such as an aluminum or copper sheet, usable as a substrate for anemitter electrode. Details of the electrospinning process employed inthe present invention are shown in FIG. 3.

Next, the nanofiber web layer formed on the substrate by electrospinningis stabilized (S17). This stabilization is achieved by annealing thenanofiber web layer in an oxidizing gas atmosphere at a predeterminedtemperature, and thus refers to “an oxidation process”. The annealingconditions vary depending on the kind of the polymer used, but theannealing is preferably carried out at a temperature ranging from about150° C. to about 350° C. for 2˜5 hours. At this step, the polymerpresent in the nanofiber web layer is crosslinked by the action ofoxygen, making the nanofiber web layer stable.

Finally, the stabilized nanofiber web layer is carbonized (S19). In step(S19), this carbonization is achieved by annealing the stabilizednanofiber web layer in an inert atmosphere at a predeterminedtemperature, and refers to a step wherein organic components remainingafter formation of a hexagonal graphite structure are removed from thecrosslinked polymer, and the crosslinked polymer is converted to acarbon fiber. The carbonization is preferably carried out in an inertgas atmosphere, e.g., nitrogen, at 600˜1,300° C. for about 0.5 hours toabout 1 hour. Since organic components remaining after formation of ahexagonal graphite structure are removed from the crosslinked polymer instep (S19), the problem encountered with remaining organic components inthe prior art using a conductive polymer can be solved. In addition,additional annealing may be carried out to activate the carbonnanofibers.

As demonstrated above, according to the method of the present invention,an emitter electrode having a carbon-fiber web structure can befabricated by electrospinning. Although the electrospinning has beenmainly employed to prepare a carbon fiber for use in fuel cells and cellelectrodes, an emitter electrode having a carbon-fiber web structure canbe fabricated by electrospinning a carbon nanotube-containing polymersolution.

In the emitter electrode having a carbon-fiber web structure accordingto the present invention, since the carbon nanotubes are fixed in thecarbon nanofibers or the carbon-fiber web structure, they have strongadhesive strength. In addition, since the emitter electrode of thepresent invention has a basic structure consisting of the highlyconductive carbon nanofibers with a large specific surface area, thecontact resistance can be greatly increased.

FIG. 3 is a schematic view of an electrospinning apparatus usable in themethod of the present invention. It will be understood that theapparatus is provided to carry out the step (S15) shown in FIG. 2.

As shown in FIG. 3, the electrospinning apparatus comprises a pipette21, a rotating drum 22, and a high-voltage generator 23 electricallyconnected to both the pipette 21 and the rotating drum 22. The carbonnanotube-containing polymer solution 24 (i.e. a mixture of the carbonnanotubes, the carbonizable polymer and the solvent) is stored in thepipette 21. The polymer solution 24 is maintained at a constant level inthe pipette 21 by an automatic feeding machine (not shown). A substrate26 on which an emitter electrode is to be formed is mounted on therotating drum 22. The rotating drum 22 is connected to an electric motor28 such that the rotating drum 22 is rotated at a constant speed.

When an electric voltage generated by the high-voltage generator 23 isapplied between the polymer solution 24 present in the pipette 21 andthe rotating drum (or the substrate 26), the polymer solution 24 is spunon the substrate through a spinneret 25 so that nanofibers 27 having adiameter of a few nanometers to hundreds of nanometers are formed on thesubstrate 26. The nanofibers 27 form a randomly twisted web structure.Each of the nanofibers 27 constituting a nanofiber web contains carbonnanotubes a few nanometers to tens of nanometers in length. A portion ofthe carbon nanotube is exposed during subsequent stabilization orcarbonization, thereby enabling the fabrication of a carbon nanotubeemitter structure with a desired shape.

The diameter of the spinneret 25 of the pipette 21, the distance betweenthe pipette 21 and the rotating drum 22 (or the substrate 26), theapplied voltage, and the rotation speed of the rotating drum 22 arecrucial processing factors in the electrospinning process. These factorscan be appropriately selected by a person skilled in the art. It ispreferred that the diameter of the nanofibers is larger than that of thecarbon nanotubes so as to allow the nanofibers to contain the carbonnanotubes.

The electrospinning apparatus is only one embodiment usable in thepresent invention. Any electrospinning apparatus that can spinnanofibers from the polymer solution may be used in the electrospinningprocess.

Hereinafter, the present invention will be explained in more detail withreference to the following example.

However, since this example is provided for illustrative purposes onlyand the kind of starting materials and other processing conditions canbe appropriately changed by a person skilled in the art, it is not to beconstrued as limiting the scope of the invention.

EXAMPLE 1

First, about 70 wt % of polyacrylonitril (PAN) as a polymer wasdissolved in DMF to prepare a polymer solution. Separately, 0.5 wt %, 1wt %, 3 wt %, 5 wt %, and 10 wt % of multi-wall carbon nanotubesprepared by CVD were mixed with the polymer solution to prepare fivecarbon nanotube-containing polymer solutions.

By using an apparatus (spinneret diameter: 0.5 mm) similar to theelectrospinning apparatus shown in FIG. 3, each of the carbonnanotube-containing polymer solutions was spun on a copper film as asubstrate to form five nanofiber web layers thereon. At this time, theapplied voltage was 20 KV, and the distance between the spinneret of thepipette and the substrate was 10 cm.

Next, each of the nanofiber web layers was oxidized at about 250° C. for3 hours with oxygen blowing. FIGS. 4 a to 4 e represent nanofiber webstructures formed after the oxidation (or stabilization). Specifically,these figures are SEM images (5,000×) of nanofiber web structures formedfrom the polymer solutions in which 0.5 wt %, 1 wt %, 3 wt %, 5 wt % and10 wt % of the carbon nanotubes were contained, respectively. It couldbe confirmed from the images that the nanofibers have a diameter rangingfrom about 300 nm to about 500 nm.

To determine whether or not the carbon nanotubes were well contained inthe respective nanofiber structures, the nanofiber structures weremagnified. FIG. 5 is a high magnification SEM image of the nanofiber webstructure (formed from the polymer solution in which 10 wt % of thecarbon nanotubes were contained) of FIG. 4 e. As shown in FIG. 5, twostrands of carbon nanotubes (CNTs) (diameter: about 10 nm) are exposedat one end of a nanofiber located in the center of the image. Thispartial exposure of nanotubes indicates that the structure can act as afield emitter electrode.

Next, the oxidized (or stabilized) nanofibers were annealed undernitrogen atmosphere at about 900° C. for 45 minutes. The annealing wascarried out to completely remove organic components remaining afterformation of a hexagonal graphite structure. As a result, the nanofiberweb layer was composed of carbon fibers.

FIGS. 6 a through 6 c are photographs of the carbon nanofiber web layer(formed from the polymer solution in which 10 wt % of the carbonnanotubes were contained) taken at different magnifications. Referringto FIGS. 6 a and 6 b, it could be confirmed that basic structures aremaintained, despite partial deformation resulting from the removal oforganic components remaining after formation of a hexagonal graphitestructure.

FIG. 7 is a photograph showing the luminescence state of a fieldemission device to which the carbon-fiber web layer is applied. As canbe seen from FIG. 7, it was observed by visual examination that arelatively uniform luminescence occurred in the overall areas. This isbecause a portion of the carbon nanotube is exposed to the outside ofthe nanofiber, as shown in FIG. 5, and a voltage is applied to thecarbon nanotube through the electrically conductive carbon nanofiber toshow field emission effects.

As demonstrated in this example, the use of the electrospinning processenables the fabrication of a carbon nanotube emitter electrode capableof improving the adhesive strength of carbon nanotubes while showingsuperior luminescence properties.

It should be understood that the scope of the present invention is notlimited by the foregoing embodiments and the accompanying drawings, butis defined by the claims that follow. Accordingly, those skilled in theart will appreciate that various substitutions, modifications andchanges are possible, without departing from the technical spirit of thepresent invention as disclosed in the accompanying claims, and suchsubstitutions, modifications and changes are within the scope of thepresent invention.

As apparent from the above description, the field emitter electrodehaving a carbon-fiber web structure according to the present inventionis fabricated by mixing carbon nanotubes, a carbonizable polymer and asolvent to prepare a carbon nanotube-containing polymer solution, andelectrospinning the polymer solution. Since the emitter electrode of thepresent invention not only improves the adhesive strength of carbonnanotubes but also exhibits improved contact resistance, it showssuperior luminescence properties.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A field emitter electrode having a carbon-fiber web structure,comprising: a carbon-fiber web layer composed of a plurality of carbonnanofibers; and a plurality of carbon nanotubes adhered to or containedin the plurality of nanofibers, wherein at least a portion of the carbonnanotube is exposed to the outside of the nanofiber.
 2. The fieldemitter electrode according to claim 1, wherein the carbon-fiber weblayer is formed of at least one material selected from the groupconsisting of polyacrylonitriles, celluloses, phenol resins, andpolyimides.
 3. The field emitter electrode according to claim 1, whereinthe carbon nanofibers have a larger diameter than the carbon nanotubes.4. A method for fabricating a field emitter electrode having acarbon-fiber web structure, comprising the steps of: mixing acarbonizable polymer, carbon nanotubes and a solvent to prepare a carbonnanotube-containing polymer solution; electrospinning the polymersolution to form a nanofiber web layer on a substrate; stabilizing thenanofiber web layer such that the polymer present in the nanofiber weblayer is crosslinked; and carbonizing the nanofiber web layer such thatthe crosslinked polymer is converted to a carbon fiber.
 5. The methodaccording to claim 4, wherein the carbonizable polymer is at least oneselected from the group consisting of polyacrylonitriles, celluloses,phenol resins, and polyimides.
 6. The method according to claim 4,wherein the nanofiber web layer is composed of nanofibers having adiameter larger than the carbon nanotubes.
 7. The method according toclaim 4, wherein the substrate is an electrically conductive substrate.8. The method according to claim 4, wherein the stabilization of thenanofiber web layer is carried out by oxidizing the polymer present inthe nanofiber web layer in an oxidizing atmosphere at 150˜350° C.
 9. Themethod according to claim 4, wherein the carbonization of the nanofiberweb layer is carried out by carbonizing the crosslinked polymer in aninert atmosphere at 600˜1,300° C.